Once you have created the required cache coordination classes, for more information on configuring a user-defined coordinated cache, see [[Configuring%20a%20Custom%20Coordinated%20Cache%20(ELUG)|Configuring a Custom Coordinated Cache]].

Once you have created the required cache coordination classes, for more information on configuring a user-defined coordinated cache, see [[Configuring%20a%20Custom%20Coordinated%20Cache%20(ELUG)|Configuring a Custom Coordinated Cache]].

Cache Coordination

The need to maintain up-to-date data for all applications is a key design challenge for building a distributed application. The difficulty of this increases as the number of servers within an environment increases. EclipseLink provides a distributed cache coordination feature that ensures data in distributed applications remains current.

Cache coordination reduces the number of optimistic lock exceptions encountered in a distributed architecture, and decreases the number of failed or repeated transactions in an application. However, cache coordination in no way eliminates the need for an effective locking policy. To effectively ensure working with up-to-date data, cache coordination must be used with optimistic or pessimistic locking. We recommend that you use cache coordination with an optimistic locking policy (see Configuring Locking Policy).

You can use cache invalidation to improve cache coordination efficiency. For more information, see Cache Invalidation.

Cache Isolation

Isolated client sessions provide a mechanism for disabling the shared server session cache. Any classes marked as isolated only cache objects relative to the life cycle of their client session. These classes never utilize the shared server session cache. This is the best mechanism to prevent caching as it is configured on a per-class basis allowing caching for some classes, and denying for others.

Cache Locking and Transaction Isolation

By default, EclipseLink optimizes concurrency to minimize cache locking during read or write operations. Use the default EclipseLink transaction isolation configuration unless you have a very specific reason to change it.

Cache Coordination

As the following figure shows, cache coordination is a session feature that allows multiple, possibly distributed, instances of a session to broadcast object changes among each other so that each session's cache is either kept up-to-date or notified that the cache must update an object from the data source the next time it is read.

When sessions are distributed, that is, when an application contains multiple sessions (in the same JVM, in multiple JVMs, possibly on different servers), as long as the servers hosting the sessions are interconnected on the network, sessions can participate in cache coordination. Coordinated cache types that require discovery services also require the servers to support User Datagram Protocol (UDP) communication and multicast configuration (for more information, see Coordinated Cache Architecture).

Cache coordination reduces the occurrence of stale data by increasing the likelihood that distributed caches are kept up-to-date with changes and are notified when one of the distributed caches must update an object from the data source the next time it is read.

Cache coordination reduces the number of optimistic lock exceptions encountered in a distributed architecture, and decreases the number of failed or repeated transactions in an application. However, cache coordination in no way eliminates the need for an effective locking policy. To effectively ensure working with up-to-date data, cache coordination must be used with optimistic or pessimistic locking. We recommend that you use cache coordination with an optimistic locking policy (see Configuring Locking Policy).

For other options to reduce the likelihood of stale data, see Stale Data.

Coordinated Cache Architecture

EclipseLink provides coordinated cache implementations that perform discovery and message transport services using various technologies including the following:

Message transport services allow the session to broadcast object change notifications to other sessions participating in cache coordination when a unit of work from this session commits a change.

Descriptor

You can configure how object changes are broadcast on a descriptor-by-descriptor basis. This lets you fine-tune the type of notification to make.

For example, for an object with few attributes, you can configure its descriptor to send object changes. For an object with many attributes, it may be more efficient to configure its descriptor so that the object is flagged as invalid (so that other sessions will know to update the object from the data source the next time it is read).

Unit of Work

Only changes committed by a unit of work are subject to propagation when cache coordination is enabled. The unit of work computes the appropriate change set based on the descriptor configuration of affected objects.

Coordinated Cache Types

JMS Coordinated Cache

For a JMS coordinated cache, when a particular session's coordinated cache starts up, it uses its JNDI naming service information to locate and create a connection to the JMS server. The coordinated cache is ready when all participating sessions are connected to the same topic on the same JMS server. At this point, sessions can start sending and receiving object change messages. You can then configure all sessions that are participating in the same coordinated cache with the same JMS and JNDI naming service information.

Because you must supply the necessary information to connect to the JMS Topic, a JMS coordinated cache does not use a discovery service.

If you do use cache coordination, we recommend that you use JMS cache coordination: JMS is robust, easy to configure, and provides efficient support for asynchronous change propagation.

For more information on configuring JMS, refer to your see JMS provider's documentation.

RMI Coordinated Cache

For an RMI coordinated cache, when a particular session's coordinated cache starts up, the session binds its connection in its naming service (either an RMI registry or JNDI), creates an announcement message (that includes its own naming service information), and broadcasts the announcement to its multicast group (see Configuring a Multicast Group Address and Configuring a Multicast Port). When a session that belongs to the same multicast group receives this announcement, it uses the naming service information in the announcement message to establish bidirectional connections with the newly announced session's coordinated cache. The coordinated cache is ready when all participating sessions are interconnected in this way, at which point sessions can start sending and receiving object change messages. You can then configure each session with naming information that identifies the host on which the session is deployed.

CORBA Coordinated Cache

For a CORBA coordinated cache, when a particular session's coordinated cache starts up, the session binds its connection in JNDI, creates an announcement message (that includes its own JNDI naming service information), and broadcasts the announcement to its multicast group (see Configuring a Multicast Group Address and Configuring a Multicast Port). When a session that belongs to the same multicast group receives this announcement, it uses the naming service information in the announcement message to establish bidirectional connections with the newly announced session's coordinated cache. The coordinated cache is ready when all participating sessions are interconnected in this way, at which point, sessions can start sending and receiving object change messages. You can then configure each session with naming information that identifies the host on which the session is deployed.

Currently, EclipseLink provides support for the Sun Object Request Broker.